If the front drive shaft is turning, but the front wheels are not. (when vehcle is raised)Test for engine vacuum in and out of the transfer case vacuum switch when in 4HI.If OK, check for servo actuation.The front axle servo is located at the RF engine compartment.
If you have vacuum to the servo and there are no vacuum leaks in the servo diaphram (it will hold vacuum) then look for the control cable from the servo to the front axle to be rusted and broken.
REMOVAL & INSTALLATION
See Figures 1, 2 and 3
Fig. 1: Tube-to-frame attachment
Fig. 2: Apply grease to hold it in position, then install the thrust washer to the output shaft tube
Fig. 3: Checking the shift mechanism
The following procedure requires the use of the shift cable housing seal Installer tool No. J-33799 or equivalent.
Disconnect the battery cables (negative cable first), then remove the battery and battery tray.
Disconnect the shift cable from the vacuum actuator by disengaging the locking spring. Then push the actuator diaphragm in to release the cable.
Unlock the steering wheel at steering column so the linkage is free to move.
Raise and support the front of the truck safely using jackstands.
Remove the front wheel assemblies.
If equipped, remove the front axle skid plate.
If necessary, remove the right side caliper and support using safety wire.
Place a support under right-side lower control arm and if equipped, unload the torsion bar.
Disconnect right-side upper ball joint, then remove the support so the control arm will hang free.
To keep the axle from turning, insert a drift through the opening in the top of the brake caliper, into the corresponding vane of the brake rotor.
Matchmark the right-side drive axle to the output shaft. Remove the right-side drive axle shaft-to-tube assembly bolts and separate the drive axle from the tube assembly, then remove the drift from the brake caliper and rotor.
Unplug the 4WD indicator lamp electrical connector from the switch.
Remove the three bolts securing the cable and switch housing-to-carrier and pull the housing away to gain access to the cable locking spring. DO NOT unscrew the cable coupling nut unless the cable is being replaced.
Disconnect the cable from the shift fork shaft by lifting the spring over the slot in the shift fork.